1. Field of the Invention
The present invention relates to an electrostatic actuator with an interdigitated electrode structure for use in an optical device for switching an optical path or adjusting a quantity of light coupled to each incident port by inserting/extracting a mirror into/out of an optical path.
2. Description of the Related Art
FIG. 8 shows a structure of a MEMS (Micro Electro Mechanical Systems) optical switch disclosed in Japanese Patent Application Laid-Open No. 2005-37885 as an exemplary prior art structure of an optical device of this sort using the electrostatic actuator with the interdigitated electrode structure.
Four fiber grooves 1A, 1B, 1C and 1D are formed into a shape of a cross by being coupled from each other at each one end on an upper face 20u of a substrate 20 made from a silicon wafer for example. The four optical fibers 41A, 41B, 41C and 41D are disposed within the corresponding fiber grooves 1A, 1B, 1C and 1D, respectively.
A straight slot 25 that makes an angle of 45 degrees respectively with the fiber grooves 1A and 1B is formed on the upper face 20u in a driver forming section 20A that is an area between the fiber grooves 1A and 1B that make right angles from each other. Then, a movable rod 11 that is provided with a mirror 4 at one end 11a thereof is placed within the slot 25. The movable rod 11 is movable along the straight slot 25 in this case. Each one end of leaf spring-like hinges 23R, 23L, 24R and 24L is anchored to both sides of two supporting points 111 and 112 spaced apart on the movable rod 11 and each other end of the hinges 23R, 23L, 24R and 24L is anchored to the substrate 20. The movable rod 11 is linked to the substrate 20 through each hinge as a bridge member and is supported movably in a longitudinal direction thereof and in parallel with a plane (the upper face 20u) of the substrate 20. The hinges 23R, 23L, 24R and 24L are mounted so that they elastically warp in the same direction. The movable rod 11 assumes a first stable state in the state shown in FIG. 8 and the mirror 4 is positioned at the center of the radially disposed optical fibers 41A, 41B, 41C and 41D. The movable rod 11 also assumes a second stable state in which the mirror 4 is stored within the slot 25 when the movable rod 11 is driven in a direction separating from an intersection 80 of the fiber grooves 1A, 1B, 1C and 1D and the warp of the hinges 23R, 23L, 24R and 24L is reversed. Thus, the movable rod 11 carries out a so-called bi-stable operation.
Movable interdigitated electrodes 12R and 12L are mounted on the both sides of the movable rod 11 at an intermediate position between the two supporting points 111 and 112 on the movable rod 11. The movable interdigitated electrodes 12R and 12L have beams 12aR and 12aL anchored to the movable rod 11 at right angles to the longitudinal direction thereof and movable electrode fingers 12bR and 12bL mounted to the both sides of each beam 12aR, 12aL and arrayed apart from each other in parallel. Fixed interdigitated electrodes 21R, 21L, 22R and 22L are provided so as to sandwich the movable interdigitated electrodes 12R and 12L in the longitudinal direction of the movable rod 11. The fixed interdigitated electrodes 21R and 22R have fixed electrode fingers 21bR and 22bR arrayed in parallel from each other in a direction orthogonal to the longitudinal direction of the movable rod 11 and anchored to the substrate 20. Each one end of the fixed electrode fingers 21bR and 22bR is disposed so as to interdigitate with the movable electrode finger 12bR from each other. The same applies to the fixed interdigitated electrodes 21L and 22L.
The movable electrode fingers 12bR and 12bL enter most deeply into gaps between the fixed electrode fingers 21bR and 21bL and almost come out of gaps between the fixed electrode fingers 22bR and 22bL in the first stable state of the movable rod 11 as shown in FIG. 8. In contrary to that, the movable electrode fingers 12bR and 12bL almost come out of the gaps between the fixed electrode fingers 21bR and 21bL and enter most deeply into the gaps between the fixed electrode fingers 22bR and 22bL in the second stable state of the movable rod 11.
Voltage may be supplied to the movable interdigitated electrodes 12R and 12L via terminals 15R and 15L, the hinges 23R and 23L and the movable rod 11. Voltage may be supplied to the fixed interdigitated electrodes 21R and 21L via terminals 13R and 13L and voltage may be supplied to the fixed interdigitated electrodes 22R and 22L via terminals 14R and 14L. When voltage is applied between the terminals 15R and 15L (and/or between terminals 16R and 16L) and the terminals 14R and 14L in the first stable state shown in FIG. 8, the movable interdigitated electrodes 12R and 12L are attracted by electrostatic attractive force in a direction of interdigitating with the fixed interdigitated electrodes 22R and 22L and the movable rod 11 is driven in a direction separating from the intersection 80 of the fiber grooves 1A, 1B, 1C and 1D, falling into the second stable state. As a result, the mirror 4 is stored within the slot 25. When voltage is applied between the terminals 15R and 15L (and/or between the terminals 16R and 16L) and the terminals 13R and 13L in the second stable state, the movable rod 11 returns to the first stable state shown in FIG. 8. The movable rod 11, the hinges 23R, 23L, 24R and 24L, the movable interdigitated electrodes 12R and 12L and the fixed interdigitated electrodes 21R, 21L, 22R and 22L compose the actuator 100 together with the substrate 20.
By the way, although the electrostatic actuator with the interdigitated electrode structure of the optical switch described above has been already put into practical use, there has been a case when the movable rod 11 displaces in the direction crossing with the longitudinal direction thereof in the transition process between the first and second stable states when the driving voltage is applied, causing a short-circuit of the applied voltage as the movable electrode finger 12bR contacts with the fixed electrode finger 21bR or 22bR or the movable electrode fingers 12bL contacts with the fixed electrode finger 21bL or the 22bL.
It may be caused by the following reasons. That is, it is difficult to form the hinges, fixed electrode fingers, movable electrode fingers and others strictly symmetrically about a center axis line of the movable rod in fabricating the electrostatic actuator and a very slight asymmetry is produced even when no driving voltage is applied. Thereby, the electrostatic attractive force between the fixed electrode fingers and the movable electrode fingers in one direction orthogonal to the center axis line of the movable rod becomes greater than that in the opposite direction and an asymmetrical operation is induced during the transition between the both stable states when the driving voltage is applied. When the movable electrode fingers displace even a bit in one direction as a result, the electrostatic attractive force in that direction sharply increases, causing a collision of the movable electrode fingers against the fixed electrode fingers.
The operation stops when the movable electrode fingers contact with the fixed electrode fingers. Or, there is even a possibility that the short-circuited electrode fingers are fixed with each other.
In order to avoid such a phenomenon, Non Patent Literature (Rob Legtenberg et al., “Comb-drive actuators for large displacements” J. Micromech, Microeng. 6, 1996, pp. 320-329) disclosed a method of reducing the displacement caused by the electrostatic attractive force by increasing a thickness of the hinges to enhance its rigidity. When the rigidity of the hinge is enhanced, however, it becomes necessary to increase the driving voltage necessary for driving the movable rod in its center axis direction. Still more, the electrostatic attractive force also increases when the driving voltage is increased, it becomes difficult to assure a stable operation of causing no contact.